Nowadays, a large number of the drying plants for products whereof the drying procedure involves the use of air flows at increasing temperatures, either for affording a thorough drying or for effecting a heat treatment (malt, bricks, tiles, etc.) have a cyclic operation, this requiring high variations of thermal power and temperature while the treatment is taking place. Moreover, the plant, of the so-called batchwise type, is ineffective during the loading and unloading steps, viz. for about 15% of the time.
As a rule, the present design of the plants intended for the above-mentioned products can hardly be adapted for the use of heat pumps due to the very high air flows involved and the substantial differences in the temperature between the withdrawn air and the air forced in. As a matter of fact, the ratio between the amount of energy supplied and that consumed by a heat pump, which is called the performance factor, is inversely proportional to the difference between the temperature at which the heat is recovered (at the evaporator) and the temperature at which the heat is restituted (at the condenser).
In some present plants a source of heat at high temperature is provided in "series" downstream of the heat pump condenser for affording the high temperatures at which air is to be forced into the plant.
While profitable, the applications of heat pumps come up against a limitation of the performances and require substantial investments, both of these factors leading to relatively long amortization times.